In general, in the map data to be included in navigation apparatuses, a road network is converted into electronic data indicated by a line (link) representing each road and a connection point (node) of roads. In the navigation apparatuses, the processing of determining the currently traveled road on the basis of, for example, the travel distance and the traveling direction of a vehicle is performed with consideration given to errors caused by such conversion and errors in the vehicle position information (see, for example, Patent Document 1).